Existing implantable cardioverter defibrillators (ICDs) are typified by a relatively large size that usually requires implantation of the prosthetic device in the abdominal cavity of a human patient. In order to allow for effective subcutaneous implantation of a prosthetic device in the pectoral region of a human patient, the maximum size of the prosthetic device needs to be less than about 40-90 cc, depending upon the physical size and weight of the patient. Unfortunately, all existing ICDs have total displacement volumes of at least 110 cc or greater and energy storage capacities of at least 2.0 Amphours. Even though there are numerous advantages to developing an ICD having a displacement volume small enough to permit implantation of the device in the pectoral region of a human patient, to date it has been difficult to develop a practical ICD having a total displacement volume of less than about 100 cc.
For reasons of simplicity and compactness, existing ICDs are universally capacitor-discharge systems that generate high energy cardioversion/defibrillation countershocks by using a low voltage battery to charge a capacitor over a relatively long time period (i.e., seconds) with the required energy for the defibrillation countershock. Once charged, the capacitor is then discharged for a relatively short, truncated time period (i.e., milliseconds) at a relatively high discharge voltage to create the defibrillation countershock that is delivered through implantable electrode leads to the heart muscle of the human patient.
One of the primary reasons why capacitor-discharge ICDs of a smaller volume have not been developed to date relates to the electrical requirements for storing the high energy cardioversion/defibrillation countershocks that are currently used to defibrillate human patients. Cardioversion countershocks have delivered energies of between about 0.5 to 5.0 Joules and are used to correct detected arrhythmias, such as tachycardia, before the onset of fibrillation. Defibrillation countershocks, on the other hand, have delivered energies greater than about 3.0 Joules and are use to correct ventricular fibrillation or an advanced arrhythmia condition that has not responded to cardioversion therapy.
Presently, all capacitor-discharge ICDs are designed such that the capacitor can store a maximum electrical charge energy of at least about 35 Joules. In contrast, implantable pacemakers, which currently have displacement volumes of less than 50 cc, are designed to deliver pacing pulses of no more than about 50 .mu.Joules. The requirement that a capacitor-discharge ICD be capable of storing an electrical charge with enough energy to deliver an electrical pulse almost one million times as large as that of an implantable pacemaker significantly increases the size of the ICD over the size of the pacemaker due to the size of the electrical components necessary to store this amount of electrical charge energy.
The accepted requirement that ICDs be capable of storing a maximum electrical charge energy of at least about 35 Joules arises out of the definition of an appropriate safety margin for the device according to a clinically developed defibrillation success curve as shown in FIG. 11. The defibrillation success curve plots the percentage probability of successful defibrillation for a ventricular fibrillation of about 5-10 seconds versus the energy of a monophasic defibrillation countershock as measured in Joules. The safety margin for a given device for a given patient is presently accepted to be the difference between the maximum electrical charge energy (E.sub.c) stored by the capacitor in that device and the median defibrillation threshold energy (DFT) required for that patient.
Under existing medical practice, each time an ICD is implanted in a human patient, an intraoperative testing procedure is attempted in order to determine the median DFT for that patient for the particular electrode lead combination which has been implanted in the patient. The intraoperative testing procedure involves inducing ventricular fibrillation in the heart and then immediately delivering a defibrillation countershock through the implanted electrode leads of a specified initial threshold energy, for example, 20 Joules for a monophasic countershock. If defibrillation is successful, a recovery period is provided for the patient and the procedure is usually repeated a small number of times using successively lower threshold energies until the defibrillation countershock is not successful or the threshold energy is lower than about 10 Joules. If defibrillation is not successful, subsequent countershocks of 35 Joules or more are immediately delivered to resuscitate the patient. After a recovery period, the procedure is repeated using a higher initial threshold energy, for example, 25 Joules. It is also possible that during the recovery period prior to attempting a higher initial threshold energy, the electrophysiologist may attempt to lower the DFT for that patient by moving or changing the electrode leads.
The intraoperative testing procedure is designed to accomplish a number of objectives, including patient screening and establishing a minimum DFT for that patient. Typically, if more than 30-35 Joules are required for successful defibrillation with a monophasic countershock, the patient is not considered to be a good candidate for an ICD and alternative treatments are used. Otherwise, the lowest energy countershock that results in successful defibrillation is considered to be the median DFT for that patient. The use of the lowest energy possible for a defibrillation countershock is premised on the accepted guideline that a countershock which can defibrillate at a lower energy decreases the likelihood of damage to the myocardial tissue of the heart. For a background on current intraoperative testing procedures, reference is made to M. Block, et al., "Intraoperative Testing for Defibrillator Implantation", Chpt. 3; and J. M. Almendral, et al., "Intraoperative Testing for Defibrillator Implantation", Chpt. 4, Practical Aspects of Staged Therapy Defibrillators, edited by Kappenberger, L. J. and Lindemans, F. W., Futura Publ. Inc., Mount Kisco, N.Y. (1992), pgs. 11-21.
Once the median DFT for a patient is established, the electrophysiologist will determine a safety margin for a given ICD device usually by subtracting the median DFT from the maximum E.sub.c stored by that device. Alternatively, a different calculation for the safety margin is sometimes determined by estimating that point on the defibrillation success curve where the electrical energy of a defibrillation countershock will insure a 99% success (E.sub.99). Under either definition, the safety margin needs to be large enough to accommodate upward deviations along the defibrillation success curve. Such deviations may be expected, for example, with subsequent rescue defibrillation countershocks delivered later in a treatment after initial cardioversion or defibrillation countershocks of lesser energies were not successful. In these situations clinical data has found that, when delivered after 30 to 40 seconds of ventricular fibrillation, the electrical energy necessary to achieve effective defibrillation may increase 50% or more over the median DFT. As a result, an electrophysiologist usually will require that a given ICD have a first type of safety margin that is typically a factor of at least 2 to 2.5 times the median DFT for that patient before the electrophysiologist will consider implanting the given ICD in that patient. For the alternate E.sub.99 point safety margin, the electrophysiologist will require that a given ICD have a maximum E.sub.c at least 10 Joules above the E.sub.99 point.
Based on current clinical data that the average median DFT is somewhere between 10-20 Joules for a monophasic countershock, the lower limit for the maximum E.sub.c that must be stored by the ICD is accepted to be at least about 35 Joules, and more typically about 39 Joules, in order to generate a maximum defibrillation countershock having an adequate safety margin. The accepted lower limit for the maximum E.sub.c of at least 35 Joules is supported by clinical evaluations, such as Echt, D. S., et al., "Clinical Experience, Complications, and Survival in 70 Patients with the Automatic Implantable Cardioverter/Defibrillator", Circulation, Vol. 71, No. 2:289-296, February 1985. In this article, the authors evaluated data for early AICD devices having maximum E.sub.c energies of 32 Joules stored in a 120 .mu.F capacitor with a discharge voltage V.sub.d of 750 Volts. In analyzing the clinical data for minimum DFTs, the authors concluded that the 32 Joule device had insufficient energy for effective defibrillation. It should be noted that in the next generation of the particular AICD devices studied, the maximum E.sub.c for the device (the CPI Ventak.RTM.) was increased to 39.4 Joules by increasing the capacitance value of the ICD by using a 140 .mu.F capacitor.
Unfortunately, the requirement that an ICD be capable of storing a maximum E.sub.c of this magnitude effectively dictates that the size of the ICD be greater than about 100 cc. This relationship between the maximum E.sub.c that is required for an ICD and the overall size of the ICD can be understood by examining how an ICD stores the electrical energy necessary to deliver a maximum defibrillation countershock.
The only two components that impact on the ability of a capacitor-discharge ICD to store a maximum E.sub.c are the capacitor and the battery, which together occupy more than 60% of the total displacement volume of existing ICDs. Thus, it will be apparent that the size of a capacitor-discharge ICD is primarily a function of the size of the capacitor and the size of the battery. For a capacitor, the physical size of that capacitor is principally determined by its capacitance and voltage ratings. The higher the capacitance value, the larger the capacitor. Similarly, the physical size of a battery is also principally determined by its total energy storage, as expressed in terms of Amp-hours, for example. Again, the higher the Amp-hours, the larger the battery. With these concepts in mind, it is possible to evaluate how a maximum E.sub.c affects the size of the capacitor and the size of the battery in an ICD.
The maximum electrical charge energy (E.sub.c) of an ICD is usually defined in terms of the capacitance value (C) of the capacitor that stores the charge and the discharge voltage (V.sub.d) at which the electrical charge is delivered as defined by the equation: EQU E.sub.c =0.5*C*V.sub.d.sup.2 (Eq. 1)
The maximum electrical charge energy (E.sub.c) can also be defined in terms of how the energy is transferred from the battery to the capacitor. In this case,, E.sub.c is determined by the charging efficiency (e.sub.c) of the circuitry charging the capacitor, the battery voltage (V.sub.b), the battery current (I.sub.b) and the charging time (t.sub.c) as defined by the equation: EQU E.sub.c =e.sub.c *V.sub.b *I.sub.b *t.sub.c (Eq. 2)
When Eqs. 1 and 2 are used to calculate a maximum E.sub.c to be stored by the device, the capacitance value (C) and the charging time (t.sub.c) end up being the only true variables in these equations because the remaining values are all effectively determined by other constraints. In Eq. 1, for example, the discharge voltage (V.sub.d) for present ICDs can be no more than about 800 Volts due to voltage breakdown limitations of high power microelectronic switching components. As a result, V.sub.d is typically between 650-750 Volts. In Eq. 2, it will be found that, for batteries suitable for use in an ICD, the maximum battery output voltage (V.sub.b) for ICDs is typically less than 6 Volts and, due to internal impedances within these batteries, the maximum battery current (I.sub.b) is about 1 Amp. In addition, the charging efficiencies (e.sub.c) of existing ICDs are presently on the order of about 50%.
When Eqs. 1 and 2 are evaluated for any given maximum E.sub.c, it will be found that there necessarily is a minimum capacitance value (C.sub.min) for the capacitor and a minimum charging time (t.sub.min) required to store that maximum E.sub.c in the capacitor of the ICD. Knowing E.sub.c and V.sub.d, Eq. 1 can be reworked as follows to solve for C.sub.min : ##EQU1##
Similarly, knowing E.sub.c, V.sub.b, I.sub.b, and e, Eq. 2 can be reworked as follows to solve for t.sub.min : ##EQU2##
In other words, the fact that all ICDs presently use a maximum E.sub.c of at least 35 Joules means that all existing ICDs will require capacitors of greater than 124 .mu.F, and that all existing ICDs which draw 1 Amp of current from the battery will have a charging time of greater than 12 seconds. Because the physical size of the capacitor is directly proportional to the capacitance rating of the capacitor in farads for a fixed voltage, the requirement that the capacitor be at least 124 .mu.F is effectively a minimum size limitation on the capacitor for discharge voltages of less than about 800 Volts. Similarly, the requirement that each charging time for a defibrillation countershock draw at least 12 Amp-seconds of current from the battery is also a constructive minimum size limitation on the battery. Thus, it can be seen that the existing requirement for a maximum E.sub.c of at least about 35 Joules effectively dictates the size of both the capacitor and the battery and, consequently, the size of the ICD.
While existing ICDs have been successful in defibrillating human patients, and thereby saving lives, these devices are primarily limited to implantation in the abdominal cavity due to their relatively large size of greater than 110 cc. It has long been recognized that it would be advantageous to reduce the total displacement volume of an ICD sufficiently to allow for subcutaneous implantation of the device in the pectoral region of human patients. This can only be done, however, so long as the device provides for a sufficient safety margin to insure its effectiveness. Accordingly, it would be desirable to provide for an arrangement and configuration of the internal components of a capacitor-discharge ICD such that the total displacement volume of the ICD is reduced, while a sufficient safety margin for the device is retained.